1. Field of the Invention
Apparatuses consistent with the present invention relate to a flat panel type near field light generating device which includes a flat panel type solid immersion mirror to generate near field light, and a heat assisted magnetic recording head with the same device.
2. Description of the Related Art
Research has been conducted to increase the recording density of magnetic recording media. As recording density increases, the bit size for recording unit information on a magnetic recording medium is reduced. However, as the bit size is reduced, the magnetic field signal from the magnetic recording medium becomes small. Thus, it becomes necessary to reduce noise in order to secure a good signal-to-noise ratio when reproducing data. As the noise is mainly generated from a magnetization transition region of the recording medium, the signal-to-noise ratio is secured by reducing the size of the grains forming the recording bits in order to reduce the transition noise.
To stably maintain the information recorded in a magnetic recording medium, each spinning grain should maintain its recorded orientation without heat fluctuation. For this purpose, the ratio of magnetic anisotropy energy to heat energy (KUV/KBT) should have a sufficiently large value (about 60 or more), where, KU is the magnetic anisotropy energy density of the magnetic recording medium, V is the grain volume size, KB is the Boltzmann constant, and T is the absolute temperature. To secure the thermal stability of a high-density recording medium having a small grain, the magnetic anisotropy energy density of the magnetic recording medium should increase at a constant temperature. The magnetic anisotropy energy density is proportional to the coersive force materials, therefore a large coersive force should be used for the magnetic recording medium.
However, when these materials are used to increase the thermal stability of the magnetic recording medium, data recording is impossible due to the strength limit of the magnetic field generated in a magnetic recording head.
To address the aforementioned problem, a heat assisted magnetic recording system has been developed. According to this system, heat is locally applied to the magnetic recording medium to lower the coersive force so that the magnetic recording medium can be easily magnetized by the magnetic field applied from the magnetic recording head. Although the grain size of the recording medium decreases when the heat assisted magnetic recording head is used, the thermal stability is secured.
A method of irradiating laser beam is generally used to apply heat locally to a magnetic recording medium.
FIG. 1 is a schematic perspective view of a conventional heat assisted magnetic recording head 1, which comprises a recording unit for converting information into a magnetic signal and applying the magnetic signal to a magnetic recording medium 2, a reproducing unit including a reproduction element 9 for detecting a bit recorded on the magnetic recording medium 2, and an optical system including a light source 6, such as a laser diode, for heat assistance. The recording unit includes a recording pole 3, for applying a magnetic field to the magnetic recording medium 2; a return pole 4 for forming a magnetic circuit together with the recording pole 3; and an induction coil 5 for inducing a magnetic field in the recording pole 3. When the magnetic recording medium 2 moves in direction A, a light spot 7 is formed on the magnetic recording medium 2 by the laser beam generated from the light source 6, and when the coersive force is lowered immediately after the magnetic recording medium 2 is heated by the laser beam, the magnetic recording medium 2 is magnetized by magnetic flux leakage generated from the recording pole 3. The information recorded in this manner is reproduced by the reproduction element 9, which may be a giant magnetoresistance element.
To enable the high-density recording by the heat assisted magnetic recording head 1, the light spot formed by the laser beam has to be very small. For example, a light spot having a diameter of about 50 nm is required in order to obtain a recording density of 1 Tb/in2. There have been research conducted on a heat assisted magnetic recording method to obtain a small light spot by using near field light. Such research generally utilizes near field light having an aperture of the size same as or less than half the wavelength of light. However, in such a near field light head, the transmission efficiency of light is very low and it is difficult to form the aperture into the magnetic head.
U.S. Pat. No. 6,594,430 discloses a heat assisted magnetic recording head using a SIL (solid immersion lens) in the form a mode index waveguide lens, which is a two-dimensional optical element. In this patent, however, since the optical element includes a number of refraction surfaces, it is difficult to control scattering and mode conversion of a progressive wave generated at the interface between the waveguide core and the lens, it is difficult to form a large number of apertures, aberration generated due to a beam misaligned with respect to an optical axis is large, and the focal length remarkably changes according to a change of the thickness of the thin film.